Ultrasonic welding is an industrial technique whereby an ultrasonic welding device or welder locally applies high-frequency ultrasonic acoustic vibrations to workpieces that are held together under pressure to create a weld. It is commonly used for welding thermoplastic workpieces. An ultrasonic welding process includes placing contiguous or overlapped workpieces on a nest or anvil and employing an ultrasonic welder to direct high frequency vibration to an interface between the workpieces. An ultrasonic welder preferably includes a converter or piezoelectric transducer, a booster and a sonotrode, also referred to as a horn. The three elements are tuned to resonate at the same ultrasonic frequency, which may be, by way of example, 20, 30, 35 or 40 kHz. The converter converts an electrical signal into a mechanical vibration, the booster modifies the amplitude of the vibration, and the sonotrode clamps the workpieces and applies mechanical vibration thereto to join the workpieces. An electronic ultrasonic generator delivers a high power AC signal with frequency matching the resonance frequency, and is preferably controlled by a controller controlling movement of the welder of the press and delivering the ultrasonic energy. The ultrasonic vibrations generate heat that locally melts materials of the workpieces to form a weld joint.
An ultrasonic welder has a rated maximum welding force. However, known ultrasonic welders deliver vibrational energy to workpieces while clamping the workpieces together with the welder tip. When there are gaps between workpieces, the welding force may be insufficient to effectively clamp the workpieces for ultrasonic welding. Relying on the welding force to close gaps between workpieces may be inefficient because the workpieces may vibrate excessively and with energy delivered outside of an intended weld zone.